This invention relates to electric motors and in particular, to an electric motor incorporating a speed sensor.
The emphasis on designing electric motors, especially miniature permanent magnet electric motors, has been and continues to be to reduce the size or volume of the motor without loss of power output. However, in the past, as components are added to a motor to improve an aspect of the motor, the size of the motor was increased to accommodate the new components. This is particularly evident for speed sensors which are required for servo systems. Speed sensors generally provide a voltage or pulse output whose amplitude or frequency is a function of the speed of the motor. This signal is used by a servo amplifier to control the motion of the motor.
Typical techniques to measure the speed of a motor include adding tacho-generators or frequency generators to the shaft of the motor. These require additions to the motor frame size and thereby increases its bulk. Techniques involving detecting the current peaks in the motor""s electrical input have been tried and while moderately successful, involve sensitive electronics which are affected by noisy environments and unclean commutator switching which often occurs with motor wear rendering the sensor inoperative in a worst case scenario.
Accordingly, there is a need for a speed sensor which is less dependent on the input power and which does not increase the bulk or size of the motor.
This is achieved by the present invention by a sensor coil being formed on an inner face of a stator magnet and located in the air gap between the magnet and the rotor.
Thus, the present invention, in one aspect thereof, provides a permanent magnet direct current motor comprising: a permanent magnet stator including at least one permanent magnet; a rotor including a rotor shaft, an armature core mounted on the shaft and having a plurality of poles, an armature winding wound about the poles, and a commutator mounted on the shaft adjacent one end of the armature core and connected to lead wires of the armature winding, the rotor being journaled in bearings and located confronting the stator; and a speed sensor; wherein the speed sensor is a coil of conductive material fixed to a surface of the magnet and located in the air gap between the permanent magnet and the armature core.
Preferably, the coil is a single turn coil fixed to an inner face of the magnet facing the armature core.
Preferably, the coil is a single turn coil in the form of a long narrow xe2x80x9cUxe2x80x9d.
Preferably, the terminals of the coil are located on an axial end face of the magnet.
Preferably, the terminals of the coil mate with resiliently deformable fingers or spring biased terminals fixed to the motor end cap. The deformable fingers may be metallic strips or conductive rubber terminals fixed to the end cap.